The present invention relates to a protective system for a three phase high voltage transmission line, and more particularly, to an overvoltage directional blocking relay having means to more correctly sense the presence of an external fault, i.e., a fault behind the relay location.
As the voltage and current ratings and the length of transmission lines have increased, greater emphasis has been placed on decreasing the operating times for the protective relays associated with such lines so as to provide shorter fault clearing times. The desired relay operating times may be relatively small compared to the time constants of the natural frequency transients on long high voltage transmission lines.
In a directional comparison relaying system for a transmission line, there are typically directional relays at opposite terminals of the line, and each comprises a tripping unit and a blocking unit. If the tripping unit at one line terminal senses fault current flowing into the transmission line through this one terminal, it will act to produce tripping of a circuit breaker at this terminal unless the tripping unit is blocked from producing such tripping by a blocking signal received from the relay at the opposite terminal of the line. The primary purpose of the blocking unit of the relay system at the opposite terminal is to transmit a blocking signal if the fault that produced the fault current is external to the transmission line. If the fault is an internal fault, no blocking signal is transmitted and the tripping unit at the one end is free to produce tripping in response to the fault.
Since operation of the tripping unit must usually be delayed until the blocking unit of the relay at the opposite line terminal has had an opportunity to determine whether the fault is internal or external and to transmit a blocking signal if the fault is external, it is important to high speed tripping of the circuit breaker on internal faults, and hence minimum delay time, that such blocking unit be capable of quickly establishing whether the fault is an external one.
For long line applications, certain difficulties are encountered in quickly establishing the presence of an external fault. Some of these difficulties result from the fact that immediately following fault occurrence, the fault current in the line is the current associated with travelling waves propagated into the line by the fault. The steady-state fault current, which is the current that the usual blocking relay responds to, is established only after some time has elapsed relative to the initiation of the travelling waves; and this can delay response of the blocking relay to an external fault.
An object of my invention is to provide a blocking relay of the overvoltage directional type which is capable of correctly responding very rapidly to an external fault, even in long line applications where travelling waves may persist for a relatively long period.
Another condition that can cause problems for directional relays is the failure of one or two phase potentials on a three phase system. Such a failure typically causes the presence of negative sequence voltage and the absence of negative sequence current, which conditions, in turn, may cause tripping relays to operate erroneously.
A further problem that may result from slow operation of the blocking relays is one that arises in those applications where the relays are fed from parallel current transformers and the fault current is such as to produce a reasonable probability of current transformer saturation. If the prefault residual flux levels in the current transformers are such that an adversely located current transformer saturates rapidly (whereas another current transformer saturates slowly); then, if the operating times of the blocking relays are delayed, the adversely located current transformer may saturate before the blocking relays operate, creating the possibility that the lack of operation of the blocking relays will allow the trip relays to operate and cause a false trip.
A further difficulty is involved with blocking relays utilizing negative or zero sequence components in that the operation may be degraded when a fault is located on a parallel line such that the negative (and/or the zero) sequence voltage at each end of the protected line is approximately equal. Thus, the negative (and/or zero) sequence relay current at each end of the protected line is established by the negative (and/or zero) sequence voltage at the line terminals and the net shunt impedance in the line, which is typically capacitive. The capacitive shunt impedance produces a current in the relay that may cause the external fault in the parallel line to appear as an internal fault in the protected line to a negative (or zero) sequence directional unit.
Accordingly, it is an object of the present invention to provide a directional overvoltage relay utilizing the negative (or zero sequence) components having means to correctly distinguish the fault current associated with external fault conditions.
It is a further object of my invention to provide more reliable detection of external faults where the current transformers are driven into their undesirable saturation condition.
A still further object of my invention is to provide a means for detecting potential failure of one or two phase potentials on a three phase system and initiating transient blocking of the tripping relays.
It is a still further object of this invention to provide a directional overvoltage relay that is minimally affected by travelling waves on long lines, and is capable of fast operation in the presence of said travelling waves.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.